The present invention relates to a semiconductor laser device, and more particularly, it relates to a semiconductor laser device applicable to two wavelengths of a red region and an infrared region.
Currently, digital video discs (DVDs) capable of high density recording and having large capacity and DVD apparatuses for recording/reproducing data in/from DVDs are commercially available, and they are regarded as products expected to be in larger demand from now on. Since data is recorded in a high density in a DVD, an AlGaInP-based semiconductor laser of an emission wavelength of 650 nm is used as a laser light source for the recording/reproducing. Therefore, in an optical pickup of a conventional DVD apparatus, a CD-R cannot be reproduced by using an AlGaAs-based semiconductor laser of an emission wavelength of 780 nm.
Therefore, an optical pickup including lasers of two wavelengths by incorporating an AlGaInP-based semiconductor laser of an emission wavelength of a 650 nm band and an AlGaAs-based semiconductor laser of an emission wavelength of a 780 nm band into different packages as laser chips has been employed. Thus, an apparatus capable of reproducing data from any of a DVD, a CD and an MD has been realized.
The aforementioned optical pickup has, however, a large size because the two packages of the AlGaInP-based semiconductor laser and the AlGaAs-based semiconductor laser are both included. Therefore, a DVD apparatus using such an optical pickup unavoidably has a large size.
On the contrary, as described in Japanese Laid-Open Patent Publication No. 11-186651, an integration type semiconductor light emitting apparatus including a plurality of kinds of semiconductor light emitting devices of different emission wavelengths and having light emitting structures formed by growing semiconductor layers on one substrate is known.
An example of such a conventional integration type semiconductor light emitting apparatus is shown in FIG. 12. As shown in FIG. 12, in a conventional integration type semiconductor laser device 100, an AlGaAs-based semiconductor laser LD1 of an emission wavelength of a 700 nm band (of, for example, 780 nm) and an AlGaInP-based semiconductor laser LD2 of an emission wavelength of a 600 nm band (of, for example, 650 nm) are integrated on one n-type GaAs substrate 101 to be spaced from each other.
In this case, for example, a substrate having the (100) plane direction or having a plane inclined by 5 through 15 degrees from the (100) plane as the principal plane is used as the n-type GaAs substrate 101.
Also, in the AlGaAs-based semiconductor laser LD1, an n-type GaAs buffer layer 111, an n-type AlGaAs cladding layer 112, an active layer 113 with a single quantum well (SQW) structure or a multiple quantum well (MQW) structure, a p-type AlGaAs cladding layer 114 and a p-type GaAs capping layer 115 are successively stacked in this order on the n-type GaAs substrate 101.
An upper portion of the p-type AlGaAs cladding layer 114 and the p-type GaAs capping layer 115 are formed in a stripe shape extending along one direction. An n-type GaAs current confining layer 116 is provided on both sides of such a stripe-shaped portion, and thus, a current confining structure is formed. A p-side electrode 117 is provided on the stripe-shaped p-type GaAs capping layer 115 and the n-type GaAs current confining layer 116, and the p-side electrode 117 is in ohmic contact with the p-type GaAs capping layer 115. As the p-side electrode 117, for example, a Ti/Pt/Au electrode is used.
In the AlGaInP-based semiconductor laser LD2, an n-type GaAs buffer layer 121, an n-type AlGaInP cladding layer 122, an active layer 123 with the SQW structure or the MQW structure, a p-type AlGaInP cladding layer 124, a p-type GaInP intermediate layer 125 and a p-type GaAs capping layer 126 are successively stacked in this order on the n-type GaAs substrate 101.
An upper portion of the p-type AlGaInP cladding layer 124, the p-type GaInP intermediate layer 125 and the p-type GaAs capping layer 126 are formed in a stripe shape extending along one direction. An n-type GaAs current confining layer 127 is provided on both sides of such a stripe-shaped portion, and thus, a current confining structure is formed. A p-side electrode 128 is provided on the stripe-shaped p-type GaAs capping layer 126 and the n-type GaAs current confining layer 127, and the p-side electrode 128 is in ohmic contact with the p-type GaAs capping layer 126. As the p-side electrode 128, for example, a Ti/Pt/Au electrode is used.
Furthermore, on the back surface of the n-type GaAs substrate 101, an n-side electrode 129 is provided to be in ohmic contact with the n-type GaAs substrate 101. As the n-side electrode 129, for example, an AuGe/Ni electrode or an In electrode is used.
Moreover, the p-side electrode 117 of the AlGaAs-based semiconductor laser LD1 and the p-side electrode 128 of the AlGaInP-based semiconductor laser LD2 are respectively soldered onto heat sinks H1 and H2 provided on a package base 200 to be electrically separated from each other.
In the conventional integration type semiconductor laser device 100 having the aforementioned architecture, when a current is allowed to pass between the p-side electrode 117 and the n-side electrode 129, the AlGaAs-based semiconductor laser LD1 is driven. Also, when a current is allowed to pass between the p-side electrode 128 and the n-side electrode 129, the AlGaInP-based semiconductor laser LD2 is driven. In this case, a laser beam of the wavelength of the 700 nm band (of, for example, 780 nm) can be taken out by driving the AlGaAs-based semiconductor laser LD1, and a laser beam of the wavelength of the 600 nm band (of, for example, 650 nm) can be taken out by driving the AlGaInP-based semiconductor laser LD2. It is determined by switching an external switch whether the AlGaAs-based semiconductor laser LD1 or the AlGaInP-based semiconductor laser LD2 is to be driven.
In this manner, since the conventional integration type semiconductor laser device 100 includes the AlGaAs-based semiconductor laser LD1 of the emission wavelength of the 700 nm band the AlGaInP-based semiconductor laser LD2 of the emission wavelength of the 600 nm band, a laser beam for a DVD and a laser beam for a CD or an MD can be independently taken out. Therefore, when the integration type semiconductor laser device 100 is included as a laser light source in an optical pickup of a DVD apparatus, data can be recorded/reproduced in/from any of a DVD, a CD and an MD.
Since the AlGaAs-based semiconductor laser LD1 and the AlGaInP-based semiconductor laser LD2 have the laser structures made of the semiconductor layers grown on the same n-type GaAs substrate 101, the integration type semiconductor laser device can be contained in one package. Therefore, the optical pickup can be made compact, and hence, the DVD apparatus can be made compact.
Furthermore, a high optical output of a semiconductor laser is necessary for rapidly rewriting data in an optical disc. For example, in order to rewrite data in an optical disc of a DVD at a high speed exceeding a 4-time speed, a high output of 100 mW or more is necessary as the optical output. In order to obtain such a high output, it is necessary to prevent COD (catastrophic optical damage), that is, a phenomenon that the end face of the semiconductor laser is melt fractured by its own optical output in a high-output operation.
In order to prevent the COD, it is effective to suppress heat generation by reducing optical density within the end face of a resonator of the laser. As a known method employed for this purpose, the reflectance of a front end face of the semiconductor laser is lowered by coating the front end face, from which a laser beam is taken out, with a dielectric such as SiO2, Al2O3 or amorphous Si.
In general, the reflectance on the resonator end face of a semiconductor laser device made of an AlGaInP-based material or an AlGaAs-based material is approximately 30% when the end face is not coated. In this case, approximately 30% of a laser beam is reflected on the resonator end face so as to be fed back to the inside of the resonator, and the remaining approximately 70% of the beam is taken out from the front end face.
On the contrary, when the front end face is coated with a dielectric film so as to attain reflectance of, for example, 10%, 10% of the laser beam is reflected on the resonator end face to be fed back to the inside of the resonator, and the remaining 90% of the beam is taken out from the front end face.
Specifically, in the case where a beam taken out from the front end face has the same optical output, the optical density on the resonator end face can be made ⅓ by lowering the reflectance on the front end face to ⅓. Accordingly, the lowering of the reflectance on the front end face leads to increase of a COD level, and hence is effective means for obtaining a high-output laser. Furthermore, when the reflectance on a rear end face disposed on the opposite side to the resonator end face from which a laser beam is taken out is set to be high, the efficiency for taking out light from the front end face of the semiconductor laser can be further increased.
In this manner, in a high-output semiconductor laser, end face coating conditions for lowering the reflectance on the front end face and obtaining high reflectance on the contrary on the rear end face, such as an end face coating condition for attaining low reflectance of, for example, 10% or less on the front end face and attaining high reflectance of 85% or more on the rear end face, are widely employed. When such an anti reflection (AR)/high reflection (HR) coating is provided, the external differential quantum efficiency (slope efficiency) of the current-optical output characteristic is improved, so that a high optical output can be realized with a small quantity of injected current. This coating prevents the occurrence of the COD by reducing the power density of the laser beam on the front end face during the operation.
Also in a dual-wavelength laser device in which semiconductor lasers respectively lasing in the red region and the infrared region are integrated on one substrate, the front end faces and the rear end faces of light emitting portions respectively for emitting red light and infrared are coated with dielectric films capable of simultaneously attaining low reflectance and high reflectance.
Another example of the background of the invention is disclosed in Japanese Laid-Open Patent Publication No. 64-61084.